Charge pumps are used in integrated circuits to provide a boosted supply voltage in applications such as eDRAM memory, FLASH memory, bandgap voltage references, and hearing aids. Typical boost circuits first charge a capacitor from an external supply and then transfer the stored charge into a filter capacitor on a boosted supply net. A ripple voltage develops on the boosted supply net that can be minimized by large filter capacitors (dcap) and by the use of multiphase pumps.
In a 4-phase system, a voltage comparitor controls the activation of a group of 4-pumps which fire at 90-degree intervals of a clock. The charge output of a group of voltage pumps could be adjusted by enabling or disabling a number of pumps, or adjusting the number of phases used to activate the system, but as the number of phases increases, so does the number of phase dividers, boosted clock drivers and associated wiring. Regulation can be improved by deactivating excess pumps, however, charge delivery in a 4-phase system would be uneven unless groups of 4-pumps were added and subtracted as needed. An oversized dcap would also improve regulation at the expense of chip size.
In an eDRAM application, the current or charge requirement of a boosted word line (or wordline) voltage VPP does not scale linearly with supply voltage Vdd. As supply voltage Vdd drops to 0.75V (volts), for example, the charge pump system must supply nearly 100% of the current which is needed at 1.1V supply. The charge pump system must therefore be sized to provide essentially the same load current, at minimum and maximum input supply voltages.
A charge pump system can be sized to provide adequate current at low supply voltage. However, it will also typically have excessive ripple voltage unless the dcap is sized for the higher charge transfer at the maximum supply voltage. Dcap typically consumes close to 50% of the total pump area in the design and has a significant impact on chip size. A solution is therefore needed where dcap area can be reduced and charge capacity can be tailored to the load requirement.
In compilable memory systems used with charge pump systems, a charge pump system is designed for the worst case anticipated load. However, this oversized charge pump causes excessive ripple on smaller configurations having smaller loads.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.